The present invention relates to black non-magnetic composite particles for black toner and a black toner using the black non-magnetic composite particles, and more particularly, to black non-magnetic composite particles for black toner which can exhibit not only an excellent flowability but also a high volume resistivity capable of preventing the deterioration in charge amount of the black toner even when a large amount of the black non-magnetic composite particles are contained in the black toner, and a black toner using such black non-magnetic composite particles.
In conventional electrophotographic developing processes, a black toner prepared by mixing and dispersing non-magnetic black pigments such as carbon black in a binder resin, has been widely used as a developer. Recent developing systems have been generally classified into one-component developing methods and two-component developing methods.
In the two-component developing methods, the black toner and carrier are brought into frictional contact with each other to impart an electrostatic charge having a reverse sign to that of an electrostatic latent image to the black toner, so that the black toner is attached onto the surface of the electrostatic latent image due to an electrostatic attracting force therebetween, thereby neutralizing opposite electrostatic charges on the black toner and the electrostatic latent image.
On the other hand, in the one-component developing methods, since no carrier is used therein, it is not necessary to control a density of the black toner. Besides, a developing apparatus used therefor can be miniaturized due to its simple structure. However, since the one-component developing methods are inferior in developing performance or quality to the two-component developing methods, high techniques have now been required to obtain the same developing performance or quality as those of the two-component developing methods. As one of the one-component developing methods, there is known a so-called insulated non-black toner developing method using a high-resistant or insulated black toner prepared by dispersing carbon black fine particles in a binder resin without using magnetic particles.
In the case where the black toners used in the above two-component developing method and the insulated non-black toner developing method, are applied to a currently predominant PPC system of copiers, both types of the black toners are desired to exhibit a good insulating property or a high resistance, specifically to have a volume resistivity as high as not less than 10.sup.12 .OMEGA..multidot.cm.
Also, it is known that the movement of a developer in a developing apparatus is strongly governed by the flowability thereof, for example, the flowability of the developer has strong influences on the frictional charging properties between the black toner and the carrier in the case of the two-component developing method, or on the charging property of the black toner on a sleeve in the case of the one-component developing method. Recently, with the enhancement in image quality such as image density, or tone gradation or in developing speed in the developing apparatus, it has been strongly demanded to increase the flowability of the black toner.
With the recent tendency of reducing a particle size of the black toner, it has been more strongly required to enhance the flowability thereof.
With respect to such a fact, in "Recent Electrophotographic Developing System and Comprehensive Data Collection for Development and Utilization of Toner Materials", published by Japan Scientific Information Co., Ltd. (1985), page 121, it has been described that " . . . , there is shown a relationship between image definitions obtained by using various toners. As is apparent from Table 1, the smaller the particle size of wet toner, the higher the image definition becomes. When a dry toner is used, it is also required to reduce the particle size of the toner for enhancing the image definition . . . . , it has been reported that by using toners having a particle size of 8.5 to 11 .mu.m, fogs on a background area as well as toner consumption can be reduced. Further, it has been proposed that by using polyester-based toners having a particle size of 6 to 10 .mu.m, an image quality, a charging stability and lifetime of the developer can be improved. However, when such toners having a small particle size are used, it has been required to solve many problems, e.g., those problems concerning productivity, sharpness of particle size distribution, improvement in flowability, . . . etc.".
Also, the insulated or high-resistance black toner has been required to show a high blackness and a high image density of line images and solid area images on copies.
With respect to this fact, on page 272 of the above-mentioned "Recent Electrophotographic Developing System and Comprehensive Data Collection for Development and Utilization of Toner Materials", it has been described that "Powder development is characterized by a high image density. However, the image density as well as the fog density as described hereinafter, have strong influences on image characteristics".
Further, it is necessary that the insulated or high-resistance black toners can retain a sufficient charge required for the development of latent images. Therefore, it has been strongly desired that the insulated or high-resistance black toners has a volume resistivity as high as not less than 10.sup.12 .OMEGA..multidot.cm, as described above.
With respect to this fact, on page 266 of the above-mentioned literature "Recent Electrophotographic Developing System and Comprehensive Data Collection for Development and Utilization of Toner Materials" (1985), it has been described that "When the charge amount is low, the attraction force between toner and carrier is weakened, so that the toner is separated and scattered around due to agitation in a developing zone, mechanical impact against a photosensitive member, etc., resulting in causing a so-called "fogging". Conversely, when the charge amount is too high, the toner remains attached onto the carrier, so that the amount of the toner transferred to the photosensitive member is reduced, resulting in deteriorated image density. FIG. 1 shows a Thompson's diagram showing a relationship between charge amount and image quality. . . . In general, the volume resistivity is required to be not less than 10.sup.12 .OMEGA..multidot.cm (in the case of insulated toners)".
As described above, it has been strongly desired to enhance various properties of the black toner. It is known that the black toner, especially black pigments contained in the black toner or exposed to the surface of the black toner, have large influences on developing characteristics. There is a close relationship between properties of the black toner and those of the black pigments mixed and dispersed in the black toner.
That is, since the flowability of the black toner largely depends upon surface conditions of a black pigment exposed to the surface of the black toner, it has been strongly desired that the black pigment itself can show an excellent flowability. The blackness and density of the black toner also largely depend upon those of the black pigment contained in the black toner. The larger the content of the black pigment, the higher the blackness of the black toner becomes. As described above, the insulated or high-resistance black toner is required to show an insulating property to such an extent capable of retaining a necessary charge amount, especially have a volume resistivity as high as not less than 10.sup.12 .OMEGA..multidot.cm. Further, in order to enhance the blackness of the black toner, it has been strongly desired to prevent the charge amount of the black toner from being deteriorated even when a large amount of the black pigment is contained in the black toner.
Namely, in order to obtain a black toner having a volume resistivity as high as possible, it has been strongly desired to increase a volume resistivity of the black pigment as highly as possible.
At present, as the black pigment for the black toner, there has been mainly used carbon black fine particles (Japanese Patent Application Laid-Open (KOKAI) Nos. 4-142561(1992) and 10-39546(1998)).
Thus, it has been most strongly demanded to provide a black pigment for black toner which have not only an excellent flowability but also a high volume resistivity capable of preventing a charge amount of the black toner from being deteriorated even when a large amount of the black pigment is contained in the black toner. However, black pigment which can satisfy such properties has not been obtained yet.
That is, in the case where the above-mentioned conventional carbon black fine particles are used as a black pigment for black toner, in order to produce a black toner having a volume resistivity as high as not less than 10.sup.12 .OMEGA..multidot.cm, it is required that the amount of the carbon black fine particles used therein is restricted because the carbon black fine particles exhibit an electrical conductivity. As a result, there arises such a problem that the obtained black toner shows neither a sufficient blackness nor a sufficient flowability.
Further, it has been pointed out that the carbon black fine particles have problems concerning safety and hygiene. These facts are explained below.
The carbon black fine particles themselves are a conductive material having a volume resistivity as low as not more than 10 .OMEGA..multidot.cm. Therefore, when the carbon black fine particles are used in a large amount in order to enhance a blackness of the black toner, the volume resistivity of the obtained black toner is reduced, so that the toner can be no longer used as an insulated or high-resistance toner. On the contrary, when the amount of the carbon black fine particles used is reduced from the standpoint of a high volume resistivity, the carbon black fine particles tend to be buried within each black toner particle since the carbon black fine particles have an average particle size as fine as 0.010 to 0.060 .mu.m. As a result, the amount of the carbon black fine particles exposed to the surface of each black toner particle is considerably reduced, so that there arises a problem that the obtained toner is deteriorated in flowability.
Further, since the specific gravity of the carbon black fine particles is extremely low, i.e., as low as 1.80 to 1.85, the carbon black fine particles are deteriorated in handling property. In addition, when the black toner is prepared by dispersing such carbon black fine particles in a binder resin, the bulk specific gravity of the obtained black toner becomes considerably low. Therefore, the obtained toner tends to be scattered around, and deteriorated in flowability.
Furthermore, it has been reported that substances having possible carcinorgen which are produced in the course of production of the carbon black fine particles, are disadvantageously incorporated as impurities in the carbon black fine particles. Thus, it has been pointed out that the black toner using such carbon black fine particles has a problem concerning safety.
As a result of the present inventor's earnest studies for solving the above problems, it has been found that by adhering oxide fine particles and/or oxide hydroxide fine particles comprising at least one element selected from the group consisting of Si, Zr, Ti, Al and Ce, onto the surfaces of black hematite particles or black iron oxide hydroxide particles as core particles, and then coating the fine particles or the fine particles and the exposed surface of the core particles, with methyl hydrogen polysiloxane, the obtained black non-magnetic composite particles can show not only an excellent flowability but also a high volume resistivity. The present invention has been attained on the basis of the finding.